In a typical electrophotographic process, the surface of a recording material, which comprises a relatively highly conductive support having provided thereon a photoreceptive layer formed of photoconductive zinc oxide, is uniformly negatively charged in the dark, and then an optical image of irradiance corresponding to an input object is projected on the charged photoreceptive layer. Projecting the optical image onto the charged surface causes partial discharge depending on the irradiance on the uniformly charged surface, to create an electrostatic latent image.
The latent image can be converted to a visible image by electricity detecting toner particles made acting thereon. The visible image is directly fixed on the photoconductive surface in the so-called electrofax method. On the other hand, the electrostatic latent image or the visible image can be transferred onto a desired support through charge transfer, pressure transfer, magnetic transfer, or some other transfer step, and fixed thereto.
It is required of a general copying system to produce positive copies from an original positive. Therefore, when the surface of a photoreceptive layer is used in a negatively charged condition, it is required that the electricity detecting toner particles have strong and stable positive charges. As for the liquid developer containing positively charged toner particles, various kinds have already been available in the commercial market.
However, such commercially available developers are all designed for copying line originals or halftone originals, and unsuitable for reproduction of continuous tone images. That is to say, when continuous tone images are copied using such developers, it turns out that desired image density is not achieved, and that images formed tend to have a flow defect (so-called streaking), and furthermore, deposition of toner on non-image areas (fog) tends to occur.
Developers capable of solving the above-described problems to certain extents, and providing continuous tone images of good quality are disclosed in Japanese Patent Publication No. 26594/74 (U.S. Pat. No. 4,062,789) and so on, in which semialkylamide compounds of diisobutylene-maleic acid copolymers are used as charge controlling agents.
According to our experimental results, however, such developers were found to suffer from the disadvantage that when the developer was placed in a developing apparatus and used in development procedures repeated a great number of times, e.g., not less than 1,000 times, although the toner particles in the developer did not reverse their polarity since they possessed a strongly held positive charge, the clearness of the copy images gradually decreased. The decrease in clearness of the copies obtained resulted from a decrease in the quantity of toner particles adhering to the image area. A problem with respect to adhesion strength of the images after fixation becomes insufficient also results therefrom. Therefore, when images are formed on zinc oxide-resin coated paper using the foregoing developers and the paper is employed as an offset printing plate, the problems that hydrophobic property to printing ink and printing life are both insufficient are posed. In addition, the quality of the images reproduced through a transfer step is deteriorated to a great extent.